For example, an optical measurement apparatus employing a photomultiplier tube is well known in the art for measuring weak light, such as fluorescent light emitted from a scintillator. The photomultiplier tube includes a photocathode that emits photoelectrons that correspond to the amount of light of an incident beam, and a multiplying unit for amplifying and outputting the photoelectrons emitted from the photocathode. Accordingly, the amount of light of the incident beam can be measured by counting the number of pulse currents outputted from the photomultiplier tube.
However, when this optical measurement apparatus is used to perform high-precision measurements, dark current pulses emitted due to thermal fluctuations and the like, act as noise. One technique for overcoming such a problem is a method of simultaneous measurement employing two photomultiplier tubes, wherein the output pulse is deamed effective only when the same output pulse is obtained from both photomultiplier tubes.
However, the following problems have been associated with optical measurement apparatuses that eliminate dark current pulses using the method of simultaneous measurement. First, the apparatus is complex to manufacture and large in size, since it requires two photomultiplier tubes, two high-speed processing circuits, and simultaneous counting circuits. Further, counting efficiency, or precision, drops because photons must be impinged simultaneously on both photomultiplier tubes.
Another technique and the like for detecting multiple photons simultaneously are well known in the art. In this technique, the output pulse is considered as valid only when the peak value of the output pulse is greater than or equal to a specified threshold value. However, the following problems are associated with optical measurement apparatuses that eliminate dark current pulses by providing such a threshold value. As described by Mikio Yamashita, Osamu Yura, and Yasushi Kawada in “Utilization of High-Gain First-Dynode PMTs for Measuring the Average Numbers of Photoelectrons in Weak Light and Scintillations” (Bulletin of the Electrotechnical Laboratory, Vol. 47, Nos. 9 and 10, 1988), for example, dark current pulses have nearly identical output waveforms with that of an output pulse that is generated when a single photoelectron is emitted from the photocathode. However, it is impossible to completely separate, based on output waveforms from the photomultiplier tube, an event in which a single photoelectron is emitted from the photocathode from another event in which a plurality of photoelectrons are emitted. It is necessary to set a large threshold value (for example, a value equivalent to a peak value of an output pulse for four photoelectrons) in order to eliminate dark current pulses. Hence, by setting a large threshold value, it is possible to effectively eliminate dark current pulses, but more photoelectrons will pass through uncounted, resulting in a drop in counting efficiency or a drop in precision.
In order to overcome this problem, optical measurement apparatuses, capable of separately detecting an event in which a single photoelectron is emitted and another event in which multiple photoelectrons are emitted, have been disclosed, for example, in Japanese unexamined patent application publications Nos. HEI-9-196752, HEI-9-329548, and HEI-10-227695. However, the optical measurement apparatuses disclosed therein estimate the average number of photoelectrons and the like, but are incapable of eliminating dark current pulses.